Process for making hardboard



Patented Apr. 27, 1943 UNITED STATES PATENT OFFICE 2,317,394 I rnocEssroa MAKING mannom William H. Mason, Robert M. Boehm,and George G.Simpson, Laurel, Miss., assignors to Masonite Corporation, Laurel,Miss., a corporation of Delaware No Drawing. Application November 18,1939, Serial No. 305,102

. ZClaims.

material is reduced to fiber, felted into sheets;

and the sheets are dried, and the final hardboard product obtained bymaking an application of heat and pressure. to the dried sheet for asustained period of time by means of pressing plates.

This invention is an improvement on U. S. Patent No. 2,120,137to W. H.Mason. In that patent there is but one activation of the self-bondinproperties of the ligno-cellulose fibers which is performed in the finalstep of making a sustained application of heat and pressure to a drysheet of ligno-cellulose fiber. With the present invention, however, theself-bonding properties of the ligno-cellulose fiber are activatedtwice; first, by making a material and controlled application of heattreatment, in the presence of a large percentage of moisture, to theligno-cellulose fiber or ligno-cellulose material from which the fiberis made, and, second, are again and further activated inthe dry state bythe use of heated pressing surfaces for applying consolidating pressureto a dried sheet of this fiber. Further, with the present invention, byreason of these self-bonding properties having been activated in thepreparation of the fiber, the final step of making a sustainedapplication of heat and pressure to the 'dry sheet composed of fiber soprepared can be be developed by a heat treatment which leaves thematerial in a predominantly fibrousstate adapted for giving highstrength to the'board made therefrom. Hardwoods or deciduous woods suchas the various varieties of gum, and mag- -nolia, poplar, cottonwood andthe like are so adapted to the greatest extent, also the woody fiber ofcane and similar annular growths can be used and such materials will be.grouped under self-bonding properties" and products made from I itlacking resistance to absorptionof water and wet strength.

.Neither mechanical lulose material without heat treatment, such as.used in making ground wood for example, nor

by controlled heat treatment applied in the presence of a largepercentage of moisture, and after the sheet of so-treated material. isformed and dried, the self-bonding properties of the lignocellulosefiber material are again and further ace tivated by controlledapplication of heat and pressure to the dried sheet of such materialduring the consolidation thereof into a board or other product.

Another object of the invention is the provision of a process by which agreatly improved fiber hardboard is produced from a dried sheet, ofligno-cellulose fiber in a relatively short interval of time and withoutresort to the application of excessive temperature and pressure.

A further object is the provision of a very stiff hardboard having goodproperties of dry and wet strength and water resistance, but withoutbeing necessarily made excessively thin and dense.

. To secure the advantages of the invention to the fullest extent, theligno-cellulose material taken should be of such character that theselfbonding properties of fiber made therefrom can fibration byexplosion not involving a heat treatment which will serve to activatethe self-bonding properties of the fiber throughout the whole of thematerial taken for treatment is adapted for preparing fiber materialusable for the purposes of the present invention; For example, theexplosion with steam carried out with merely such short, exposure tosteam as to permit penetration of ligno-cellulose chips followed quicklyby pressure release and explosion to obtain a fiber, which is thesubstantial equivalent of ground wood fiber as pointed out in the U. S.Patents Nos. 1,663,505 and 2,120,137 to W. H. Mason, does not involve aheat treatment of the entire charge taken for such explosion with steamwhich is adapted for preparing fiber material usable for securing thebenefits of the present invention. Fiber so exploded, ground wood fiberand the like, are hereinafter referred to as raw fiber. The table givenhereinafter shows the improvement obtained by the present invention overuse of such raw fiber.

0n the other hand, while the heat treatment must be fairly considerableinorder to activate the self-bonding properties of the ligno-cellulosefiber in the .presence of considerable proportions of moisture, itshould not be carried to such extent that the whole mass of materialtaken for treatment is converted to predominantly plastic state, as forproducing the very dense bodies described in U. S. Patent No. 2,080,078,or bodies which can be consolidated by substantially momentaryapplication of heat and pressure defibration of the ligno-cel scribed inU; S. Patent No. 2,140,189. If such predominantly plastic. material,which has little or no fibrous structure after hot pressing, were giventhe hereinafter described treatment in the press which gives goodresults with the present invention, only a brittle product would beproduced. I

Depending on the type of ligno-cellulose material, the size of thepieces of wood, such as chips,

moisture content of the raw material and other variables, the time andtemperature and the disintegration treatment are adjusted with thepresent invention so as to subject the ligno-cellulose -material to aheat and moisture treatment and disintegration adapted to secure amaterial which is predominantly fibrous and in which the selfbondingproperties of the ligno-cellulose fiber throughout the entire chargeof'material taken for treatment are activated to a substantial extent.The heat treatment, consisting in application of heat in the presence ofmoisture, is preferably performed in a closed chamber or gun by use ofhigh-pressure, preferably saturated, steam upon the ligno-cellulosematerial, which material is preferably charged into the gun in somewhatsubdivided-state, as in the form of chips about one inch or less inlength, the chips ordinarily containing a material proportion, as 25% ormore, of water.

The time and the temperature of heat treatment are somewhatinterchangeable, longer time being required at relatively lowtemperature, and

'the time being considerably decreased at higher steam temperature andpressure. The temperature should exceed about 20090., which is sublbs.per sq. in. (temperature 01' about 255 C.) in

about 85 seconds, and maintaining it at this latter pressure for about20 to 30 seconds additional, after which by opening a. small outlet theentire mass is explosively discharged from the region of high pressureto atmopsheric pressure, which disintegrates the material to fiber andterminates the action.

In order to ensure best penetration of the steam throughout thechips, itis of advantage to use lower temperature at first and then highertemperature, in which case the time of the applica tion of the highertemperature will be less than would be needed if only'the highertemperature is applied, as may be done. For example, with gumwood chipshaving about the same moisture content, good results may be obtained byadmitting saturated steam to raise the pressure from atmospheric toapproximately 500 lbs. per sq. in. (temperature of 245 C.) in about 30seconds, following this by increasing the pressure to about 1000 lbs.per sq. in. (temperature of 285 C.) in about l'second, and maintainingthis pressure for about 5 or 6 seconds followed by explosive discharge.

-It is not necessary to explosively discharge the aterial at thetermination of the heat treatment although this will stop the heattreatment quickly and secure fibration of the wood or wo dy material.The heat treatment can be terminated without explosion of the wood orwoody material, as by bleeding oi! the steam, and this heat-treatedmaterial may later be mechanically fibrated by known methods, or thematerial may be refined while under the pressure applied for the heattreatment. The heat treatment may precede, accompany or follow thedisintegration treatment so long as the'material for making the sheetsgoing to the press has undergone heat treatment as herein described orthe equivalent thereof. Little refining, or beating, if any, need begiven to: the fiber when .flbrated by explosion with steam and, if it isput through a refiner, this serves principally to secure more uniformityin' its formation into sheets.

The fibrous material obtained as described contains a considerableproportion, as about oi material soluble or dispersible in water, and

in order to secure best resistance to water absorption in the product,this material is reduced to a lesser proportion, as for example itmay be"re-.

duced to 5%, and this may be done by expression from the fiber of watercontaining water-solubles which are usable for manufacture ofbill-Products, or by treatment of the fiber with water, which can, beconveniently accomplished in the operation of forming the fiber intosheets from water, or both.

The fiber produced as described is suspended in a large proportion ofwater, as 1% of fiber by weight to 99% of water, and formed into sheetform from the water. A good grainless, felted formation can be obtainedwith this fiber, and this is of advantage in getting a. good surface anduniform high strength of the final product.

After forming into sheet form from water the sheet is preferably passedbetween squeeze rolls which reduce the water content, as toapproximately 'or less, after which the continuously formed sheet ispreferably cut into board length to be later dried and hot pressed.

While binding agents from extraneous sources are not needed, they couldbe incorporated, if

desired.

We preferably incorporate waterproofing size for enhancing theresistance to water absorption and the wet strength of the product. Inusing hydrocarbon size, such as petrolatum, for example, the water bathor stock chest may be maintained at a temperature above the meltingpoint of petrolatum, and molten petrolatum, to the extent of around 1%to 5% on the weight of the dry fiber, may be incorporated with the pulpin the water bath. Other waterproofing sizes, such as rosin size andalum, for example, may be used, and instead of maintaining the waterbath at a temperature above the melting point'of the size, which ispreferable with the use of ,petrolatum size, the waterproofing agent maybe emulsified and introduced cold, if desired, orin yet other ways. Thesizingv material, as petrolatum for example, may be put into the gunwith the chips 11' desired.

The wet sheet formed from-the water batlr'isdried, preferably completelydried, but drying to complete dryness in a drier is an expensive andprotracted operation, and after going through a drying operation whichis commercially practicable, the sheet may still contain a smallpercentage of moisture, as 1 to 2% of moisture.

This small percentage of water can be removed in the press ashereinafter described. The drying is preferably done without applicationof pressure, but pressure may be applied during the sorb as much as 7 or8% of moisture.

4 pors are permitted to escape.

drying operation if desired. When dried without pressure, the sheetsmade as ,described are ordinarily of about 0.4-0.6 sp. gr.

The sheets are preferably charged into the or explosion of the productupon opening the press while fully heated.

The press platen temperatures and the pressure required to again andfurther activate the self-bonding properties of the fiber and tothoroughly bond the heat-treated fiber together into a stiff, hard,strong, final product depend to a considerable degree on the particularwood or by sufficient roll pressure can be applied to effect someconsolidation of the sheet as to about 0.? sp. gr., for example. Thismakes a firmer sheet,

' better adapted to withstand rough handling and to avoid scufilng ofthe sheet in charging into the press, which could cause defects in thefinished board. Inasmuch as this rolling is not a finishing treatment,and the boards are to be later subjected to heat treatment in the press,the rolls, if used, need not be highly heated. A roll temperature ofabout 150'-250 C. and a roll pressure of about 40,000 lbs. per linearfoot of roll bite are ample to give a firm sheet well adapted forhandling into the press. Rolls so used for only a partial compaction ofthe sheet can be run at practically any desired speed as moisture andvapors can readily escape without injury to the sheet because of thesheet bein compacted only partially.

If the sheets are allowed to stand and pick up atmospheric moistureafter drying, they may ob- In such case, before they are charged intothe press, they are preferably redried to make them completely dry, orto reduce the moisture content to about 2% or less.

The dried sheets or blanks, whether or not partially consolidated byheated rolls, are finished by pressing in a hot press, preferably ahydraulic press, and preferably amultiple platen press, having forexample openings for hot pressing 20 sheets at a time, between heatedpressing surfaces which may consist of the press platens or surfaceplates, and subjected therein to heat and pressure for a sufficient timeto effect their final and permanent consolidation. When it is desired tomake a board smooth on both sides. both pressing surfaces are smooth,continuous and imperforate. However, vapor escape during hot pressing isnot detrimental so long as the pressing surfaces are adapted to supplyheat eff ciently to the sheets, and if, for example, only one smoothside is wanted, the other pressing surface may be faced with fine wiremesh or the like, or patterned as desired, and the surface of thefinished board will be the counterpart of the pressing surface.

It is desirable, particularly if the sheets contain residual moisturewhen charged into the press, to-at first merely close the press thereon,making little or no pressure application beyond perhaps the mere weightof the platens. Thesheets can thus be heated to substantially thetemperature of the press' platens without material pressure applicationwhile moisture or va- After a short time, as about 1-'-4 minutes, thepressure is raised to the extent required for the final bonding andconsolidation of the sheet. In this way, even though a sheet may containsome moisture when charged into the press, and even though continuousimperforate pressing surfaces be used, the final hot pressing can beperformed with no blisterin woody material used, and the amount of heattreatment which has already been given to the initial wood or woodymaterial, and the product desired. In general hard or deciduous wood ispreferred, with which a press platen temperature of at least 200 C. isdesirable to secure activation of the self-bonding properties of thefiber containing natural encrustants and obtain good bonding andconsolidation of the previously heattreated material in a reasonablyshort time.

Higher temperatures may be employed such as about 240 C. so long as thetempertaure is not sufficient to cause material loss in strength withthe time and pressure employed.

The pressures applied in the press are largely interchangeable with thetemperature and time, and may be varied to give a board product of thedesired density. Pressures of approximately 500;. per sq. in. to 1'500#per sq. in. or higher may be used, depending not only on the variablesmentioned above but also on the amount of refining which may be given tothe fiber.

, For any given time in the press, the press temperature and pressurerequired for pressing the heat-treated fiber of the present inventionare both less than those required with fiber which has not been given. aheat treatment adapted to activate the self -bonding properties. If rawfiber is used, such as ground wood or equivalent, and

substantially all the activation of the self-bonding properties of suchraw ligno-cellulose accomplished in the press alone, the sheet must begiven a large amount of heat treatment and high pressure in the press,or a relatively poor product made, since raw ligno-cellulose materialdoes not have good bonding power whenhot pressed in the absence ofmoisture unless the press temperatures and pressures are suflicientlyincreased and the times sufliciently prolonged to secure such effect.

This-comparative situation will be illustrated by the following typicalinstances, using a pressing time of 3 minutes in each case: With thefiber of wood or woody material in a raw or substantially raw state, asby being exploded with steam at 1000# per sq. in., but with little or nodwell at bonding properties of the ligno-cellulose fiber in the pressalone, and secure tolerably good consolidation and strength in 3 minutespressing time. treatment in accordance with the present invention,however, as by means of steam raised to 500# per sq. in. in 30 secondsfollowed by increasing the steam pressure to 1000# per sq. in. in

' 1 second, with a dwell of about 6 seconds at such higher pressure,followed by explosion, a product which is much stronger arid moreresistant to absorption of water is obtained in 3 minutes pressing timewith a pressing temperature of only about 210 C. and pressure of onlyabout 700# per sq. in. The following table shows the above comparison,with additional details, the fiber sheets used in each case having beenthoroughlydried before I.

With the material given a preliminary heat I pressing, and each boardcontaining about 2% of petrolatumior sizing:

V Table Heat-treated hardwood Fiber 3 min 240 C 1.1 I Modulus of rupturedry 8000! sq. in W000i sq in 24 hr. water absorption 12% 8%.

6000lsq. in.... 9000! sq m Modulus of rupture wet (after soaking 24brainwater).

With the present invention, important advantages are obtained; not onlyis the temperature and pressure and/or the pressing cycle greatlyreduced but the product secured is very much superior in substantiallyall physical properties.

1. The double-activation process of making superior hard fiber board,which comprises the steps of subjecting wood or woody ligno-cellulosematerial, composed of ligno-cellulose fibers con-' taining the naturalfiber encrustat-ion, in small pieces, to the action of steam at atemperature over 200 C. and corresponding pressure maintaining thetreatment with steam at such temperature and pressurefor a time that itis too long with relatiom to the steam temperature for production of araw fiber by means of explosion with steam, but

being much stifi'er, stronger, harder and more water-resistant than ifsubstantially raw ligno cellulose fiber material were used, and thisprodnot is welladapted for replacing higher cost products for many uses.

\ Another advantage of the process of the present invention is the factthat pressing equipment may begmuch lighter and less expensive since thehydraulic pressures required are lower, and lower steam pressures can beused for supplying steam for heating the press platens.

. Another advantase is that the heat-treated and shorter fiber may beeasily and uniformly felted on the forming machine, leading to a smoothand uniform surface on the board or other product.

A further advantage of the invention is that,

if press pressure is used as high as required with and morewater-resistant product in the final hot-pressing operation even whenusing lower pressures and temperatures and/or times in this hot-pressingoperation than where the sheets are made of raw fiber, it is apparentthat these good qualities of the final product are secured partly by thepreliminary heat treatment and partly by the final hot-pressingoperation performed upon the preliminarily heat-treated material.However, it is to beobserved that theconditions for the two heattreatments are not altogether the same, the preliminary heat treatmentbeing performed in the presence of water and followed by veryconsiderable reduction of temperature in forming the sheet butnevertheless contributing very materially to the good properties of thefinal board, whereas the final hot-pressing operation and the heattreatment in such connection is not long enough to produce a generalbreakdown of fiber structure or conversion to a state of predominantplasticity; to thereby produce a material initial activation of theself-bonding properties of the ligno-cellulose fibers, reducing thesteam treated fibrous material to greater fine ness, removing at leastpart of the water-soluble constituents, forming a sheet of the finefiber, drying the sheet, subjecting the sheet to sustained consolidatingpressure of at least 500 lbs. per sq. in. in a platen press betweenpressing surfaces heated to a temperature over 200 C. and maintainingthe application of said pressure and temperature to the sheet for aperiod of time which with pressure and temperature used is sufiicient toactivate the self-bonding properties of the ligno-cellulose fibersasecond time and produce the superior hard fiber board, the time ofsustained pressure application being approximately three minutes whenplaten, pressure 01' 700 pounds per square inch-is used with pressingsurfaces at a temperature of 210 C.

2. The double-activation process of making a superior hard fiber board,which comprises the steps of subjecting ligno-cellulose material,consisting at least in large part of deciduous wood and. with thecomponent lignocellulos'e fibers containing the natural fiberencrustation, in small pieces, to the action of high pressure steam attemperatures from about ZOO-285 C., and maintaining the steam treatmentat such temperatures for periods ranging from about 15m 20 minutes at200 C. and about 20 to 30 seconds at 255 C. (600 pounds steam pressure)to about ,5 to 6 seconds at 285 C. (1,000 pounds steam performed in theabsence or substantially in the absence of water and results immediatelyin the finished board. Accordingly it is to be understood that our useof the expression activation of self-bonding properties or equivalentexpressions in connection with each of these two heat treatments, is notto be construed as meaning that they are alike in all respects.

We claim:

pressure), which said steam treatment is too great for production of rawfiber by means of explosion with steam and is not sufficiently extensive to produce'a general breakdown of fiber structure or conversionto a state of predominant plasticity, to. thereby produce a materialinitial activation of the self-bonding properties of the ligno-cellulosefibers, reducing the steam-treated fibrous material to greater fineness,removing at least part of the water-soluble constituents therefrom,forming the fine fibrous material into a sheet, drying the sheet, andsubjecting the sheet to sustained consolidating pressure of at least 500lbs. per sq. in. in a platen press between pressing surfaces heated to atemperature over 200 C. and maintaining the application of said pressureand temperature for a period of-time which with pressure and temperatureused is sufilcient to cause the self-bonding properties of theligno-cellulose fibers to be activated a second time and to produce asuperior and highly strong hard fiber board, the time of sustainedpressure application being approximately three minutes when platenpressure of 700 pounds per square inch is used with pressing surfaces ata temperature of 210 C.

. WILLIAMH. MASON ROBERT M. BOEHM. GEORGE G. SIMPSON.

